1. Field of the Invention
The invention relates to ultraviolet light curing apparatus for curing ink which has been applied to printed stock by a screen printing apparatus or the like. In particular, the invention relates to ultraviolet curing of screen printer products which reduces heat imparted to the printed stock during curing, while improving the rate at which curing is effected.
2. Description of the Prior Art
Over the years, inks of different chemical types have been proposed for screen-printed products. After analyzing problems associated with different types of ink solvents, problems of space requirements for equipment processing the printed products, and problems of achieving commercially acceptable chemical reaction rates, ultraviolet (hereinafter "UV") inks represent the most viable approach for producing screen-printed materials in a commercial production environment.
One prominent problem associated with UV curing is the heat rise of the printed product inherent in the UV curing operation. In one aspect, heating of the printed stock is inherent in UV curing, since UV lamps which provide the source of UV curing energy require a plasma arc having a typical temperature of 2300.degree. F. Further, in order to sustain the arc within the lamp the outer envelope of the lamp, usually made of quartz glass, must be maintained at 1500.degree. F. It can be readily appreciated, therefore, that one major problem attending UV curing is that the substrate upon which the ink is printed, absorbs heat from the UV radiation source, particularly since the printed stock is close to the UV source to reduce UV losses. Commercial printing operations frequently accumulate printed material in stacks adjacent the printing station, and an excessive temperature rise in the stock is objectionable. The residual heat accumulated from a number of sheets of recently printed stock can be significant, particularly for sheets interior of the stack, where convection cooling is not available. A need therefore exists for cooler methods of UV printing, and several arrangements have been proposed for a forced cooling of the printed stock, to remove residual heat build-up. To date, these methods have proven to be the most effective for reducing the temperatures of printed products.
U.S. Pat. No. 4,434,562 discloses an ultraviolet curing apparatus for curing UV sensitive ink which has been applied to a substrate, such as a sheet of paper, paperboard stock or textile goods by a screen printing apparatus. The ink-bearing sheet is carried on a mesh conveyor through a housing in which is located one or more UV lamps which direct UV light to impinge on the ink on the upper side of the traveling sheet. The sheet is held down on the open mesh conveyor belt by means of a suction applied from a suction blower unit located beneath the belt. The suction applied also draws air through light baffles which are impervious to air. The suction forces hold the sheet flat against the mesh conveyor belt and against fluttering or otherwise flapping from the surface of the conveyor belt. A fan located on top of the housing directs cooling air over the reflector and leading portion of the stock as it exits the curing apparatus.
Another significant improvement in cooling the paper stock as well as the UV lamp is provided in U.S. Pat. No. 4,646,446, which locates a cooling station immediately downstream of the UV curing station. Air knives at the cooling station increase the air velocity, and cause a turbulent flow across the sheet to provide cooling of the sheet. An air-pervious conveyor overlying a suction device secures the sheets against fluttering at both the curing and cooling stations.
Two-stage UV curing has been proposed to provide a pretreatment of the ink before being exposed to a final source of curing radiation. In many arrangements of this type, two UV lamps are provided, one located upstream of the other, to provide a preconditioning of the ink. However, such multi-lamp arrangements are expensive to manufacture and operate, are bulkier than single-lamp units, and tend to produce more heat partly because of the duplication of energy-consuming lamp components. Considerations of space are particularly important for multicolor printing operations wherein substrates are typically loaded onto a movable conveyor apparatus which moves the substrates along a sequence of printing stations, each printing a different color ink onto the substrate. In installations of this type, curing units must be provided at each printing station to cure the ink before advancing the substrate to the next printing station. The weight of the curing stations is also important, as when the curing and printing stations are supported by a common frame.
One example of preconditioning to improve UV curing rates is given in U.S. Pat. No. 3,983,039. An arrangement is provided for reducing oxygen inhibition of intermediate chemical reactions which slow the UV polymerization of the ink. A pre-curing is employed to seal the surface layer of the uncured photosensitive ink film to reduce the effects of oxygen inhibition on the ink's deeper layers. A single lamp is used to effect the pre-curing or surface sealing of the ink at a relatively low energy level, which is achieved in a first or upstream planar reflector portion. A second or downstream reflector portion is curved to provide a peaked relatively high intensity region of UV illumination. The surface sealing of the pre-curing is accomplished with a lower level UV illumination of the ink. However, this approach ignores other mechanisms attendant in the UV curing process, and in general, significant reductions in curing rates are still being sought.
As will be discussed below, other approaches to lowering of the temperature stock by cooling the UV lamp or reflector, or by altering the shape of a given reflector, have been proposed. However, as will be discussed below, a careful review of these approaches during the initial stages of developing the invention has indicated that these approaches are, in general, ineffective to reduce the temperature rise experienced in printed stock using UV curing. Improvements in curing rates for commercial printing operations are still being sought. It is generally desirable from a system operations standpoint, that the curing station not be the limiting factor in high-speed multicolor printing operations, and any reduction in process times, such as the time required to cure light-sensitive ink contributes directly to the profitability of a printing operation.